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Re: [webrtc-pc] Semi-sync replaceTrack API pre-initial-negotiation is bizarre.

From: jan-ivar via GitHub <sysbot+gh@w3.org>
Date: Mon, 12 Feb 2018 16:38:45 +0000
To: public-webrtc-logs@w3.org
Message-ID: <issue_comment.created-364981383-1518453524-sysbot+gh@w3.org>
The reasons are coming back to me: SRD is timing-critical. If users delay calling SRD(offer), the peer connection [won't be ready to receive ICE candidates](https://stackoverflow.com/questions/36839080/webrtc-onicecandidate-am-getting-ice-candidates-with-sdpmid-audio-only-but-not/36851163#36851163).

So e.g. if users were to write:
let getMyTransceiver() = pc.getTransceivers())[0] || pc.addTransceiver("video");

signalingChannel.onmessage = async ({data: {sdp, candidate}}) => {
  if (sdp) {
    await getMyTransceiver().sender.replaceTrack(myVideoTrack); // Delays SRD!
    await pc.setRemoteDescription(sdp);
    if (sdp.type == "offer") {
      await pc.setLocalDescription(await pc.createAnswer());
      sc.send({sdp: pc.localDescription});
  } else if (candidate) await pc.addIceCandidate(candidate);
...they might occasionally miss ICE candidates, [a hard-to-find bug](https://stackoverflow.com/questions/36839080/webrtc-onicecandidate-am-getting-ice-candidates-with-sdpmid-audio-only-but-not/36851163#36851163).

Possible solutions may be to either:
 1. Enqueue replaceTrack, or
 2. Allow `sender.track = myVideoTrack` IFF "transceiver is not yet associated with a media description"

GitHub Notification of comment by jan-ivar
Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1769#issuecomment-364981383 using your GitHub account
Received on Monday, 12 February 2018 16:38:50 UTC

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